1. Field of the Invention
The present invention relates to a high-refractivity low-dispersion optical glass having a refractive index (nd) of more than 1.9, an Abbe number (νd) of at most 38 and a partial dispersion ratio (θg, F) of at most 0.615, and to an optical device such as lens and prism to be obtained by the use of the optical glass.
2. Background Art
High-refractivity low-dispersion glass is in great demand for materials for optical devices such as various lenses, etc. As an optical glass having a refractive index (nd) of more than 1.9 and an Abbe number (νd) of at most 38, known is a glass composition typically as in Patent References 1 and 2.
An optical system comprising such optical glass is mounted on optical products such as digital cameras; and for overcoming color aberration, it is desired that the high-refractivity low-dispersion optical glass has a reduced partial dispersion ratio.
For general-purpose optical glass, its cost must be low, for which, therefore, the material expense that is one factor to increase the product cost must be reduced.
For the reasons mentioned above and from the viewpoint of usefulness in optical planning, heretofore, an optical glass having characteristics of high refractivity and low dispersion and having a reduced partial dispersion ratio and capable of being produced from inexpensive materials is earnestly desired.
In particular, a high-refractivity low-dispersion optical glass having optical constants, a refractive index (nd) of more than 1.9 and an Abbe number (νd) of at most 38, is earnestly desired.
Patent References 1 and 2 disclose an optical glass having an extremely high refractive index (nd). However, the optical glass concretely disclosed in these patent references does not satisfy the requirement that it contains B2O3, La2O3, TiO2, Nb2O5 and Ta2O5 and that the ratio, as % by mass, of TiO2/Nb2O5 is at most 0.26 and GeO2/Nb2O5 is at most 0.38.    Patent Reference 1: JP-A 09-278480    Patent Reference 2: JP-A 2005-179142